Vignetting, also referred to as lens falloff, is prevalent in photography. Vignetting refers to the phenomenon of brightness attenuation away from the center of the image due to, in part, the physical dimensions of the camera optic elements such as the effective focal length of the lens system. For example, foreshortening of the lens when viewed from increasing angles from the optical axis can lead to off-axis illumination falloff or the cosine fourth law of illumination falloff. Vignetting can also be geometric in nature. For example, light arriving at oblique angles to the optical axis can be partially obstructed by the rim of the lens.
To determine the vignetting effects in an image, various approaches can utilize calibration images captured by the same camera that captured the image being analyzed, a collection of overlapping images, or other means to gain information from pixels with equal scene radiance but differing attenuation of brightness. These approaches, however, can be ineffective for use on images captured with an unknown camera or downloaded from the Internet.